2014

Got the issue this lunch. rebuilding alsa with module-assistant was simple and did the trick. I used mplayer -ao oss mod_of_the_week.ogg to validate my sound hardware was still working properly.
The issue is most likely in pulseaudio: re-installing it afterwards killed alsa sound again.

Ubuntu 13.10 (and derivatives) users: if you're using Skype downloaded from its official website and not from the Ubuntu repositories, chances are the sound in Skype is either distorted or there's no sound at all.